The U.S. Centers for Disease Control and Prevention urged Americans on Thursday to get vaccinated for measles amid an outbreak that began at Disneyland in December, saying that 2014 saw the highest number of cases in two decades.

So far more than 90 people have been diagnosed with measles in California and elsewhere, most of them linked to an outbreak that public health officials suspect began when an infected person from outside the United States visited Disneyland in Anaheim between Dec. 15 and Dec. 20.

On Wednesday, a high school in Palm Desert, California, barred dozens of non-vaccinated students from school over concerns that a classmate may have contracted the highly contagious disease.

Measles was declared eliminated in the United States in 2000 but Dr. Anne Schuchat, director of the CDC's National Center for Immunization and Respiratory Diseases, said the disease could still easily be brought in by a traveler from abroad.

“This is a wake-up call to make sure we keep measles from getting a foothold in our country,” she said.

The outbreak has renewed debate over the so-called anti-vaccination movement in which fears about potential side effects of vaccines, fueled by now-debunked theories suggesting a link to autism, have led a small minority of parents to refuse to allow their children to be inoculated.

Schuchat called it “frustrating” that some Americans had opted out of the vaccine for non-medical reasons, saying it was crucial that they be given good information about the safety and reliability of inoculations.

Asked by reporters if health officials were worried about the possibility that the outbreak could be further spread at the Super Bowl in Phoenix, Schuchat said only that people everywhere should be on their guard but declined to identify the football game on Sunday as an area of special concern.

“I wouldn't expect the Super Bowl to be a place where many unvaccinated people will be congregating,” she said.

Homegrown measles, whose symptoms include rash and fever, was declared eliminated from the United States in 2000. But health officials say cases imported by travelers from overseas continue to infect unvaccinated U.S. residents.

The sometimes deadly virus, which is airborne, can spread swiftly among unvaccinated children.

There is no specific treatment for measles and most people recover within a few weeks. But in poor and malnourished children and people with reduced immunity, measles can cause serious complications including blindness, encephalitis, severe diarrhea, ear infection and pneumonia.